OSPF Filtering (LSA Type-3 & NSSA External)

In today’s post, we will talk about the different ways we can perform filtering in OSPF.

OSPF is a Link-State protocol; hence, we can foresee that filtering is a little more difficult to perform than with any other IGP.   The reason is that link-state protocols do not advertise routes per se, they advertise topology information.   We have to keep in mind that routers in the same area have or share an identical copy of the Link-State Database (LSDB) for that particular area, this implies that OSPF filtering can only be applied at certain points of the network, being these the ABRs or ASBRs.

OSPF route filtering types:

In Cisco OSPF implementation there are three major types of OSPF filtering:

  • LSA Type-3 Filtering: This type of filtering prevents the ABR from creating particular Type-3 summary LSAs.
  • NSSA External Routes Filtering: This type of filtering prevents the P-bit (Propagate Bit) being imposed to the Type-7 LSAs, thus the Type-7 to Type-5 translation does not take place in the ABR.
  • Filtering with Summarization: This type of filtering work in conjunction with OSPF summarization, adding the not-advertise keyword.

Now, there are some cases when we can apply filtering to the router’s routing table itself, preventing routes learned from the OSPF RIB to be installed into the global RIB.  This scenario, in particular, may result into traffic blackholing because it doesn’t filter the LSA, only the route.

The OSPF RIB filtering can be applied anywhere.

LSA Type-3 Filtering

This type of filtering allows ABRs to filter Type-3 LSAs.   Remember, the ABRs do not forward Type-1 or Type-2 LSAs; instead, they create Type-3 LSAs with the values corresponding to the subnets and the cost from the ABR to these subnets.  This feature filter the type-3 LSA before is injected into another area.  Thus, the requirement of identical LSDB within the area is still met.

To configure LSA Type-3 filtering, use the area {area-source} filter-list prefix {prefix-list-name} in|out OSPF process command.   Worth mentioning that when filtering routes into the backbone area (Area 0) will filter the advertisement within all areas in the routing domain while filtering into a non-backbone area will filter the advertisement of routes only within the targeted area.

Let’s use the same topology we have been using so far but with a few changes for the examples:


Here you can download the diagram and configuration files: OSPF-Filtering – Part I

Among the changes done with respect to the topology used in the previous post, I can mention the advertisement of networks in R1, R4, R5 and R7 using sub-interfaces instead loopbacks, all routers have loopback0 configured (Not shown in the diagram). Another change is the renumbering of the transit networks and the renumbering of Area 20 to Area 40 only as Stub.

Let’s take a look to the routing table from the perspective of R1:


As can be seen in the above output, R1 is routing the eastbound traffic towards the networks in R5 and R7 through R2 and R3 (load-balancing).    The routes are being advertised as “O IA” or OSPF inter-area, meaning advertised from the ABRs as Type-3 LSAs.


Let’s apply LSA Type-3 filtering to these routes.

Goal 1:

Eastbound traffic from R1 destined to the networks in R5 must be advertised only from R3 and the networks in R7 must be only advertised only from R2.


To have a clear vision of how it works, I will describe the advertisement process.

The source of the routes to be filtered passes from Area 40 through R4 (ABR) which has OSPF summarization configured.  R4 which has the interface E0/0 into Area 40 summarize the Type-1 LSAs received from R5 and R7 and generate two Type-3 summary LSA for the summary routes ( and, R4 set itself as the advertising router for the summary LSAs in Area Zero (Backbone Area) and set the cost to reach the networks. Then this Type-3 LSAs are flooded within the area.

Next, R2 and R3 which are also ABRs generates and flood their own Type-3 LSAs for Area 30, setting themselves as the advertising router for both summary routes and adding the cost advertised from R4 to the cost from each ABR to R4.

From the perspective of R1, the Type-3 LSAs were advertised from R2 and R3 respectively and the AREA SOURCE of the advertisements is Area 0.      This means the prefix-list to be used to filter the LSAs must go out of area zero (outbound).

So, for the first part: Traffic destined to through R3, let’s apply filtering in R2 denying the summary in the outbound direction from Area 0.

Let’s take a look to the Summary LSAs of from R1’s perspective before applying the change:


As can be seen in the above output, we have two summary LSAs in R1 (Area 30) corresponding to the summary network, one LSA advertising the network from R2 (ABR), and the other from R3 (ABR) with identical cost (metric),  thus the network is load-balanced between both ABRs.   We had already established this; you may wonder why are we seeing this again?

The answer is:   To show you the effect of apply Type-3 LSA filtering in the LSDB.

Ok, let’s apply the following change in R2:

In R2 (ABR):

ip prefix-list Filter-Net-10-5 seq  5 deny
ip prefix-list Filter-Net-10-5 seq 10 permit le 32
router ospf 1
area 0 filter-list prefix Filter-Net-10-5 out

Let me explain the filter:

  1. The prefix-list Filter-Net-10-5 sequence 5 will deny the specific subnet
  2. The prefix-list Filter-Net-10-5 sequence 10 will permit any other subnet. The syntax matches any network and the le (less-or-equal) set the range of subnets lower or equal to /32.   This is the equivalent to permit any.    It is important to remember at the end of prefix-lists, access-lists or even route-maps there is an implicit deny.   Therefore without this particular entry, all subnets will be denied by default.
  3. The OSPF process command area 0 filter-list prefix Filter-Net-10-5 out, set the filter for traffic sourced from Area 0 which match the prefix-list Filter-Net-10-5 in the outbound direction.

Let’s take a look to the Summary LSAs of in R1 after we applied the change:


As can be seen in the above output, now the LSA advertised from R2 (ABR) has the delete flag set.   The LS age was changed in R2 to 3600 seconds which is the max age and the metric also was set to the max-metric for routes in the OSPF in decimal.  This is also known as LSInfinity.  Remember: When the max age timer on an LSA reaches 60 minutes, the LSA will be discarded.

Let’s take a look to the routing table in R1:


As can be seen in the output, now eastbound traffic towards R5 is following the desired path.  We still have pending the rest of the goal.    To force the traffic from R1 to R7 through R2, we have to make basically the same change, but in R3 (ABR).

So, let’s do it.

In R3 (ABR):

ip prefix-list Filter-Net-10-7 seq  5 deny
ip prefix-list Filter-Net-10-7 seq 10 permit le 32
router ospf 1
area 0 filter-list prefix Filter-Net-10-7 out

Again, let’s take a look to the routing table in R1:


Ok, as can be seen in the above output, now the eastbound traffic towards R7 is following the desired path.   With this last change, the goal 1 was achieved.

Let’s move on to the next type of filtering.

NSSA External Route Filtering

This type of filtering is applied in ASBRs.   What it does is preventing the P-bit or Propagate-Bit being imposed to the Type-7 LSAs.    Let’s recall a little bit.    NSSA generate Type-7 LSAs to advertise external routes to the ABR.  The ABR then translate the Type-7 LSAs into Type-5 LSAs.    For the translation takes place, the ABR check every Type-7 LSA looking for the P-bit (Propagate-Bit), which is imposed by default in NSSA when redistributing in ASBRs.    Now, if the P-bit is not set, then the translation does not take place.    Remember, the flooding scope of Type-7 LSAs is NSSA areas only.

We can apply this filter to the redistribution itself with the keyword nssa-only at the redistribution OSPF process command.

I will not explain mutual redistribution in this post; however I want to show you the effect on the LSDB when applying the filtering into the redistribution in OSPF.

Let’s apply NSSA external route filtering to the routes coming from the EIGRP domain.

Goal 2:

Filter the routes coming from the EIGRP domain in such way the routes will only be accessible from Area 30.

To achieve this goal, first, I would make a small change required for this requirement.   I will remove the summarization in R1.    The reason to do this is because when using summarization in conjunction with filtering applied to the redistribution process, the summarization will override the P-bit suppression.   The summarized Type-7 LSA will be generated by the ASBR with the P-bit set.

So, let’s remove the summarization and check the NSSA external database in R1.

In R1 (ASBR):

router ospf 1
no summary-address



As can be seen in the above outputs, now the LSDB has individual Type-7 LSAs per network redistributed from the EIGRP domain with the P-bit set.   The direct result of this change is that the elected Type-5/Type-7 translator (R3 ABR in this case) will generate Type-5 LSAs for each Type-7 LSA advertised by R1 and will flood them to the Backbone area, and from there to the other areas as shown in the next output from R4.   This is an expected behavior.


Now, let’s apply the NSSA external filtering adding the keyword nssa-only at the redistribution OSPF process command and observe the changes in the LSAs.

In R1 (ASBR):

router ospf 1
redistribute eigrp 100 subnets nssa-only


As can be seen in the above output, now the LSAs were generated without the P-bit.    This will prevent the translation from Type-7 to Type-5 LSAs in the ABR.   The direct result will be that the EIGRP domain will be reachable only from Area 30.

Let’s take a look to the routing table in R4 for example.


As can be seen, the routes for the EIGRP domain are not present therefore they are unreachable.

What about from routers in Area 30?   Let’s take a look to the routing table in R2 for example:


The routes are there.     In the case of R2 and R3, they are ABRs; therefore, they have an interface connecting to the NSSA area.   Both ABRs have an LSDB for Area 0 and Area 30 respectively.  Note the routes are represented as “O N2” or OSPF NSSA external routes (Type-7 LSAs) instead “O E2” or OSPF external routes (Type-5 LSAs).   This means that if we had more routers only in Area 30, they will have reachability to the EIGRP domain via OSPF as “O N2” OSPF NSSA external routes.

It is time to close this long post.  In the next one, we will continue discussing OSPF filtering with summarization and other filtering options out there.

Thank you for visiting.

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